Touch position sensors usable with finger or other stylus have been taught in U.S. patents including (i) a surface elastic wave type in prior art U.S. Pat. Nos. 3,134,099, 3,653,031, 3,673,327, 4,642,423, 4,644,100, 4,645,870, 4,700,176, 4,746,914, 4,791,416 and (ii) a plate wave type in prior art U.S. Pat. Nos. 4,665,282, 5,072,427, 5,162,618, 5,177,327, 5,243,148, 5,260,521, 5,329,070, 5,451,723, 5,573,077, 5,591,945.
Acoustic touch position sensors utilizing surface elastic wave as taught by the above-mentioned type (i) patents have a number of problems which are more readily understood when the nature of the surface acoustic wave used in these sensors is considered. If as in the above-mentioned patents, the touch plate consists of a uniform, non-piezo electric medium, and the acoustic wave is confined at or near a single surface such as an outer surface of the touch plate, the surface acoustic wave is known as a Rayleigh wave. These waves have X and Z components such that disturbed particles move elliptically in the X-Z plane. It is characteristic of these waves that the disturbance decays rapidly with depth, that is in the -Z direction, so that the wave energy is essentially confined at or near the surface of the touch plate. Strictly, Rayleigh waves exist only in an infinitely thick medium. Waves in a uniform, non-piezoelectric medium of finite thickness that are confined to a single surface as shown in FIGS. 1A-1D are more precisely termed quasi-Rayleigh waves. Given a long enough propagating path in a medium of finite thickness, quasi-Rayleigh wave energy will not be confined at or near a single surface, but will transfer back and forth between the outer surfaces of the plate. A touch sensor according to the above mentioned patents, would be inoperable under these conditions because a touch in a region of one outer surface where complete transference of the wave to the opposite outer surface has taken place, will not disturb the wave, and is therefore undetectable. In practice, in order to provide a wave that is confined to a single surface, the thickness of the touch plate must be at least three to four times the wavelength of the wave imparted into the substrate, wherein the length and breadth of the touch plate are also limited.
If the thickness of the touch plate is for example two Rayleigh wavelengths or less, the waves emanating from the source transducers utilized in the above patents are clearly distinguishable from Rayleigh/quasi Rayleigh, and other surface acoustic wave (SAW) modes and are known as Lamb waves as shown in FIGS. 1E and 1F. Lamb waves exist in two groups of various orders, all of which propagate independently of one another. One group is characterized by particle displacement that is symmetric with respect to the median plane of the plate. The other group of Lamb waves is characterized by particle displacement that is anti-symmetric with respect to the median plane. In general a specific order within the symmetric Lamb wave group differs in phase and group velocity from the identical order of the anti-symmetric Lamb wave group. In particular with a sufficient plate thickness equal to or greater than two Rayleigh wavelengths, two modes of approximately equal amplitude are mainly excited, the zeroth order symmetrical Lamb waves and the zeroth order anti-symmetrical Lamb waves. As seen in FIGS. 1E and 1F, the symmetrical and anti-symmetrical Lamb waves are not confined to a single surface of the touch plate, but extend through the plate to the opposite surface thereof. When in phase however, that is initially at and close to the source of the waves, the two Lamb waves combine to produce a quasi Rayleigh wave, as can be seen from a comparison of FIGS. 1E and 1F to FIG. 1D. As the two Lamb wave modes travel further from the source, due to the differing phase velocities and the resultant phase difference between them, there is a complete transference of wave energy from the outer surface on which the transducer, generating the wave, is mounted to the opposite outer surface. This transference of energy between the outer surfaces of the plate occurs at regularly spaced intervals, making a touch plate having large enough dimensions for this transference to occur, unsuitable for a touch position sensor.
From the above, it is seen that touch position sensors as shown in the above mentioned patents utilizing surface acoustic waves and more particularly quasi-Rayleigh waves, as is necessary for these sensors to operate, are limited to relatively thick panels, i.e., panels having a thickness of three to four times the wavelength of the surface acoustic wave propagating therein. Further, quasi-Rayleigh waves are confined at and near a single surface. The consequence of using quasi-Rayleigh waves according to the above patents leads to several undesirable attributes, namely excessive sensitivity to contaminants or other materials abutting the touch panel, and excessive panel weight and thickness for many applications. The excessive sensitivity to contamination is due to the confinement of wave energy at or near the confining surface. As a result the quasi-Rayleigh wave energy or a large fraction thereof is absorbed by even modest amounts of surface contaminants. The effect of near or total absorption of wave energy by contamination, sealant or other materials abutting the plate, is to create acoustic shadows or blind spots extending along the axes that intersect the contaminant. A touch position sensor according to the above mentioned patents cannot detect touch if one or both coordinates is on a blinded axis. In a sense, touch panels utilizing quasi-Rayleigh waves are unduly sensitive to contamination or abutting materials. The scope for optimizing the performance of a touch sensor according to the above patents is limited because touch sensitivity and minimum touch panel thickness are not independent choices. In order to support a quasi-Rayleigh wave in a touch panel of reduced thickness, its other dimensions remaining the same, the wavelength must be reduced to preserve single surface confinement. It is characteristic of Rayleigh/quasi Rayleigh waves that their confinement depth is related to wavelength, with confinement depth decreasing as the wavelength is reduced. As a result, the wave is confined to a shallower region bounded by the surface, and the proportion of wave energy absorbed by a given absorbing medium is increased. Experimentally this is found to vary, approximately by the inverse square of the wavelength. As discussed previously, touch sensors according to above patents can be considered unduly sensitive for some applications, even for relatively thick panels, hence the effect of reducing touch panel thickness results in touch sensors even more sensitive to surface contamination and other abutments. Conversely, reducing sensitivity by increasing the quasi-Rayleigh wavelength results in increased panel thickness and weight. Substantial losses in wave energy over distance as a result of air damping of the surface acoustic wave is also significant since surface acoustic waves are confined to the surface of the touch plate. The energy losses due to air damping further limit the size of the touch plate. As shown in FIGS. 1A and 1C, surface acoustic waves are imparted into a touch plate utilizing a transducer mounted on a wedge that is in turn mounted on the touch surface of the plate wherein the transducer vibrates in the direction shown to produce a compression bulk wave that propagates in the wedge to impart a surface acoustic wave in the touch plate. This type of wave generating device has several drawbacks. Because the device must convert a compressional bulk wave to a surface acoustic wave, the efficiency of the device is lower and the processing cost is higher in practice than that the transducer produced waves were of the same type as those imparted into the plate, i.e., via direct conversion. Also, because the wedge extends above the plate, it must be accommodated for in mounting the plate. Wedges are typically made of plastic thus creating a difficulty in bonding the wedge to a glass plate. Further, the transducer must be bonded to the wedge and the wedge then bonded to the touch plate. Because problems with reliability increase with the number of bonds required, this surface acoustic wave generating device is not as reliable as other wave generating devices requiring fewer bonds.
The above-mentioned disadvantage in using elastic surface wave in touch panel is the main reason that plate wave type touch panel in prior art U.S. patents were disclosed. These touch panel employing acoustic plate wave as taught in prior art U.S. Pat. Nos. 4,665,282, 5,072,427, 5,162,618, 5,177,327, 5,243,148, 5,260,521, 5,329,070, 5,451,723, 5,573,077, 5,591,945 overcome the problems imposed by the surface acoustic wave, but have their own problems. Ichiya et al's system disclosed in U.S. Pat. No. 4,665,282 requires the use of a special touch stylus capable of sensing longitudinal waves traveling across the panel.
Knowles's system disclosed in U.S. Pat. No. 5,072,427 uses Zohps shear waves as shown in FIGS. 2A-2B. and first order Lamb wave as shown in FIGS. 1E an 1F traveling across the panel. A reflective array is disposed along the first axis to convert portions of the Zohps shear wave to either first symmetric mode or first anti-symmetric mode of Lamb waves along the plurality of parallel paths extending across a touch surface of the substrate to a second reflective array the axis of which is parallel to the axis of first reflective array. Knowles's systems disclosed in U.S. Pat. No. 5,162,618, uses first order Lamb wave traveling across the panel. A reflective array is disposed along the first axis to reflect portions of the first mode Lamb waves to either first symmetric mode or first anti-symmetric mode of Lamb waves along the plurality of parallel paths extending across a touch surface of the substrate to a second reflective array the axis of which is parallel to the axis of first reflective array. Knowles's systems disclosed in U.S. Pat. No. 5,177,327, 5,243,148, 5,260,521, 5,329,070 as well as Huang et al's system disclosed in U.S. Pat. No. 5,451,723 employ Zohps shear waves traveling across the panel. A reflective array is disposed along the first axis to reflect portions of the shear wave along the plurality of parallel paths extending across a touch surface of the substrate to a second reflective array the axis of which is parallel to the axis of first reflective array. All these systems engage reflective arrays on the touch panel. it requires extra border spaces on the touch panel making it difficult for the touch panel to fit into today's display enclosure. It also requires an extra process to create the reflective array either by etching or screening process therefore increasing the cost of product as well as decreasing the yield rate during manufacturing. A small defect in the reflective arrays will cause acoustic wave diffraction as well as mode conversion which produces unwanted acoustic wave to interference with the desire acoustic waves therefore reducing the touch accuracy of the panel. Further the spacing between adjacent reflective elements along the array axis limits the touch resolution of the panel because the spacing between adjacent reflective elements along the array axis is the smallest distinguishable distance in the touch panel. On the other words, the touch panel using reflective arrays to scan the acoustic wave is still a discrete system although the touch resolution is much improved than the some systems without using arrays such as those disclosed in prior art U.S. Pat. Nos. 3,673,327 and 5,573,077.
Knowles's system disclosed in U.S. Pat. No. 5,573,077 is shown in which a number of transducers coupled to a side of a substrate impart a shear wave into the substrate for propagation along a number of paths parallel to a first axis. A reflective edge of the substrate first axis disposed along the first axis reflects the shear waves back along the parallel paths to the transducers. The transducers are responsive to the receipt of a shear wave for generating a signal representative thereof A touch on the substrate results in perturbation in the shear wave which is sensed to determine the axial position of the touch on the substrate. The major drawback of this system is that the recognizable touch points is equal to the product of numbers of transducer in one axis and the numbers of transducer in the other axis. In practice, the numbers of transducer in one axis is limited due to the size of transducer. If the size of transducer is too small, a severe diffraction occurs. Also large numbers of transducer will increase the cost of system. The transducers are energized sequentially. Each transducer has to attach an independent wire. This increases the complexity of the system significantly.